895 - Maximum Frequency Stack.

Hard

Design a stack-like data structure to push elements to the stack and pop the most frequent element from the stack.

Implement the FreqStack class:

• FreqStack() constructs an empty frequency stack.

• void push(int val) pushes an integer val onto the top of the stack.

• int pop() removes and returns the most frequent element in the stack.

Example 1:

Input

"FreqStack", "push", "push", "push", "push", "push", "push", "pop", "pop", "pop", "pop"

[ [], 5, 7, 5, 7, 4, 5, [], [], [], []]

Output: null, null, null, null, null, null, null, 5, 7, 5, 4

Explanation:

FreqStack freqStack = new FreqStack();
freqStack.push(5); // The stack is [5]
freqStack.push(7); // The stack is [5,7]
freqStack.push(5); // The stack is [5,7,5]
freqStack.push(7); // The stack is [5,7,5,7]
freqStack.push(4); // The stack is [5,7,5,7,4]
freqStack.push(5); // The stack is [5,7,5,7,4,5]
freqStack.pop(); // return 5, as 5 is the most frequent. The stack becomes [5,7,5,7,4].
freqStack.pop(); // return 7, as 5 and 7 is the most frequent, but 7 is closest to the top. The stack becomes [5,7,5,4].
freqStack.pop(); // return 5, as 5 is the most frequent. The stack becomes [5,7,4].
freqStack.pop(); // return 4, as 4, 5 and 7 is the most frequent, but 4 is closest to the top. The stack becomes [5,7].

Constraints:

• <code>0 <= val <= 10⁹</code>

• At most <code>2 * 10⁴</code> calls will be made to push and pop.

• It is guaranteed that there will be at least one element in the stack before calling pop.